Emergency illumination is designed to supply illumination and power in the event of the failure of normal supply or, in the event of accidents, to insure uninterrupted illumination where it is essential for safety to life and property. The most familiar examples are the "Exit" signs normally required in places of public assembly such as hotels, theaters, sports arenas, hospitals, office buildings, and industrial plants.
The National Electrical Code establishes certain standards for emergency illumination; and, in many if not most municipalities and states, identical or equivalent standards have been established by law. Among many other requirements, the National Electrical Code requires: (1) that emergency lighting systems be designed and installed so that the burning out of any individual lighting element cannot leave any space in total darkness; and, (2) that two or more separate and complete systems with independent power supply, each providing sufficient current for emergency lighting purposes, be supplied.
There are numerous patents related to emergency illumination apparatus - for example, U.S. Pat. Nos. 1,148,341; 1,469,946; 1,809,243; 1,875,520; 1,887,523; 2,372,124; 3,136,083; 3,309,806; 3,324,290; 3,402,494; 3,478,455; 3,587,186; 3,665,626; and 3,780,462. However, only a couple of the more recent of these patents, such as U.S. Pat. Nos. 3,916,404 and 3,931,689, either meet or could readily be adapted to meet the National Electrical Code requirements.
Hazardous atmosphere locations are locations in which the continuous or periodic presence of certain gases, vapors, dust or combustible particles creates a danger of explosion upon exposure to sparks or temperatures in excess of the ignition temperature. The National Electrical Code establishes three classes of hazardous locations: Class I locations are those in which flammable gases or vapors are or may be present in the air in quantities sufficient to produce explosive or ignitible mixtures; Class II locations are those that are hazardous because of the presence of combustible dust; Class III locations are those that are hazardous because of the presence of easily ignitible fibers or flyings, but in which such fibers or flyings are not likely to be in suspension in the air in quantities sufficient to produce ignitible mixtures.
The National Electrical Code further subdivides each of the three classes into two divisions depending upon the relative likelihood of the ignitible material being generally dispersed in the air. For example, Class I, Division 1 are those in which hazardous concentrations of flammable gases or vapors exist continuously, intermittently, or periodically under normal operating conditions. Class I, Division 2 are those in which volatile flammable liquids or flammable gases are handled, processed, or used, but in which the hazardous liquids, vapors, or gases will normally be confined within closed containers or closed systems. Although the present invention is particularly suitable for Class I, Division 1 locations, it may be advantageously employed in any hazardous atmosphere locations.
In addition, the National Electrical Code establishes four subclasses for Class I locations, Groups A-D, and three for Class II locations, Groups E-G, depending on the ignition temperature of the hazardous substances: Class I. Group A--280.degree. C. (536.degree. F.); Group B--280.degree. C. (536.degree. F.); Group C--180.degree. C. (356.degree. F.); Group D--280.degree. C. (536.degree. F.); and, Class II. Group E--200.degree. C. (392.degree. F.); Group F--200.degree. C. (392.degree. F.); and Group G--165.degree. C. (329.degree. F.). Although the present invention is particularly suitable for Class I, Groups C and D, and Class II, Groups E, F and G locations, with minor modifications as hereinafter described, this invention can be readily adapted for use under other hazardous atmosphere conditions.
The principal requirement for explosion-proof electrical apparatus operating in hazardous atmosphere locations is not that it be gas-tight but rather that the enclosure in which a spark might occur be: (1) strong enough to contain an explosion and prevent the escape of any flame; and, (2) designed so as to diffuse and dissipate any heat which could ignite the surrounding atmosphere. With regard to electrical lighting units designed for operation in hazardous atmosphere locations, these requirements are typically met by providing a light bulb surrounded by an explosion-resistant glass casing with venting for hot gases in such a manner as to prevent any flames from reaching the surrounding atmosphere.
Such designs are described, for example, in U.S. Pat. Nos. 2,574,882; 2,652,482; 2,702,849; 2,749,433; 2,770,715; 2,786,936; 2,849,597; 3,675,007; and 3,723,724. Each of these devices, however, is directed to a single lighting fixture employing a single wiring design. None of these devices, even those which might meet the hazardous location requirements of the National Electrical Code, can also meet the emergency illumination requirements of the Code. There are many instances, however, in which emergency illumination is required or at least desirable for hazardous atmosphere locations, but none of the prior art devices are capable of satisfying both sets of criteria.